Method for the control of aero gas turbine engines

ABSTRACT

This invention relates to the exchange, removal or addition, as applicable, of media and/or power between the individual shafts of an engine, between individual engines and between the engines and the aircraft. Thus, the present invention provides for additional degrees of freedom enabling engine parameters to be addressed in terms of a reduction or avoidance of negative resonances or beats. It also provides an ability to alter thrust from engines of a multi-engined aircraft to reduce rudder trim.

[0001] This application claims priority to German Patent Application10062252.6, filed Dec. 14, 2000, which application is incorporated byreference herein.

BACKGROUND

[0002] This invention relates to a method for the control of aero gasturbine engines in aircraft having at least two such engines.

[0003] In certain flight phases, some engine frequencies can excitevibrations in the aircraft, these being caused by the rotational speedof the high-pressure shaft (NH) or the low-pressure shaft (NL),respectively. These vibrations are perceived by the passengers as noiseor oscillations. Also, since the engines rarely operate at the samerotational speed, interference can occur between the engines. This givesrise to beats or standing waves. Where the rotational speed of thelow-pressure shaft is controlled, i.e. only the low pressure compressorsare synchronized, the rotational speed of the high-pressure shaft willremain a potential cause of disturbance.

[0004] Among others, the causes for different engine behavior are: Thedisparity of age between engines upon replacement of one of the engines.Inaccuracies in the measured quantities, these resulting in thegeneration of deviant controlled variables. The unavoidablemanufacturing tolerances which entail similar effects.

[0005] Normally, aero engines are both considered and controlledindividually. Only in specific cases, the interaction of aero engines istaken into consideration, for example in emergency or thrust vectoringsituations.

[0006] Few cases are known in which both engines are linked together interms of control during operation. For example, in the case of a failureof the vertical rudder, the two engines can be operated with differentthrust, this enabling turns to be flown. Also, thrust vectoring is knownin military applications (cf. U.S. Pat. No. 5,769,317 or U.S. Pat. No.6,105,901, for example).

[0007] The state of the art entails many, significant disadvantages. Itdoes not provide for the interaction between two or more engines whilemaking use of the components already available in the engines and in theaircraft. This deficiency leads to a higher noise level in the aircraftcabin. Aircraft manufacturers have to fit more attenuation material,resulting in higher mass and increased costs. Furthermore, higherinvestments have to be made into vibration reduction during aircraftdevelopment.

DESCRIPTION OF THE INVENTION

[0008] In a broad aspect, the present invention provides for avoidanceof vibrations and the resulting generation of undesired noise during theflight of an aircraft.

[0009] It is the principal object of the present invention to remedysaid problem by providing a method wherein energy, fluid and/or bleedair is removed from or supplied to at least one engine of an aircraft tocontrol the operation of the at least one engine. Further advantageousaspects of the present invention will be become apparent from thefollowing description.

[0010] Therefore, in accordance with the present invention, provision ismade for the indirect change of the engine parameters. This can beaccomplished by the removal or addition of power, energy, fluids and/orother media. As one of the possibilities, bleed air can be taken off theengine. In accordance with the present invention, said measures will notbe applied equally to all engines, but differences between theindividual engines will be permitted and induced deliberately to effectthe desired change of the engine parameters.

[0011] The present invention, therefore, enables the rotational speedsof the engines (aero gas turbines) to be changed in such a manner thatoscillations and vibrations which cause undesired noise are avoided.

[0012] As a positive effect, the resultant, additional change in thrustof the individual engines enables vertical rudder trimming to bereduced. Since no aircraft flies absolutely straight, a certain degreeof vertical rudder trimming always has to be applied. Of course, thisentails a greater aerodynamic resistance and, in consequence, impairsthe efficiency of the entire aircraft. As a further positive effect, themeasures according to the present invention, by exerting an influence onthe engine parameters, provide for compensation of differences in yaw.

[0013] Since the negative effects known in the state of the art aremostly limited to a very narrow frequency regime (resonant frequency), aminor shift of the excitation frequency (i.e. the rotational speeds) bythe measures according to the present invention can be sufficient toeffectively reduce, or completely eliminate, these negative effects.

[0014] The method according to the present invention can, for example,be implemented by the following measures:

[0015] A hydraulic power transmission (positive/negative) betweenengines, which comprise hydraulic motors/pumps, can be influenced independence of the operating conditions. Taking hydraulic power foraircraft applications from the engines to different amounts allows theengine parameters to be changed.

[0016] A hydraulic coupling of the shafts of an engine enables therotational speeds to be shifted relative to each other.

[0017] The present invention also provides for electric powertransmission (positive/negative), which, in particular, can easily beimplemented on “fully electric” engines with power exchange between theshafts and the individual engines.

[0018] A further, particularly efficient measure is the take-off ofbleed air from one of the engines.

[0019] Apparently, the present invention provides for a combination ofsaid measures and effects in order to achieve a more effective overallinfluence on a specific parameter, for example the speed of thelow-pressure shaft. Furthermore, such combination can give rise to moredegrees of freedom, this enabling secondary parameters, for example thespeed of the high-pressure shaft, to be optimized in addition to aprimary parameter, for example the speed of the low-pressure shaft. Thisis particularly advantageous in those cases where the low-pressure shaftis decisive for disturbing vibrations while some disturbing influence isexerted by the high-pressure shaft as well.

[0020] In the following, the application of the present invention isspecified for two-shaft engines. However, the present invention is alsoapplicable for engines with any number of shafts.

[0021] Effect by Hydraulic Measures

[0022] According to the state of the art, individual engines or enginegroups are operated in separate control circuits. More specifically,these control circuits are hydraulic operating circuits (e.g. foractuating the flaps or the undercarriage). In some engine designs, forexample, two hydraulic pumps supply one circuit while in others theysupply separate circuits. Depending on the arrangement and actuation ofvalves (addition of valves, if applicable), the engines can be made tocontribute a different share to the hydraulic system, i.e. their loadingand, in consequence, their parameters will change. Therefore, in thecase of two-jet aircraft, these two circuits will mostly have separatetasks. Accordingly, a power exchange between the two engines can beeffected by design changes. In the case of three-jet aircraft, thehydraulics of the third engine can be used as redundancy for the twoother hydraulic systems. Accordingly, in this case, the power parametersof the engine can also be influenced according to the present invention.On four-jet aircraft, two engines are normally connected to one controlcircuit, i.e. a power change in terms of hydraulic loading can be usedto effect a change of the power parameters of the engine also in thelatter case.

[0023] A hydraulic coupling of the various shafts of an engine enablesboth rotational speeds (high-pressure shaft and low-pressure shaft) tobe influenced (NL=f(NH)). In the function, NL indicates the rotationalspeed of the low-pressure shaft and NH indicates the rotational speed ofthe high-pressure shaft.

[0024] Effect by Electricity

[0025] The statements made in the above for the hydraulics apply almostsimilarly to electricity. However, in the case of electricity, thetake-off of different power from the two engines can be effected muchmore easily. In the case of “fully electric” engines, power exchange ofthe individual engines and of shafts between engines can be accomplishedvery simply.

[0026] Effect by Customer Bleed

[0027] Normally, bleed air is tapped during the entire flight, thisbleed air being fed by both engines into a common system. If thepressure loss between the point of tapping and mixture is different inthe bleed-air systems of either engine, the mass flows will varyaccordingly between the two engine systems. This variation willinfluence either of the two engines in a different manner and willfinally result in minor speed changes which are utilizable for theeffect according to the present invention.

[0028] Therefore, in accordance with the present invention, differentconditions are produced in the individual bleed-air systems of theengines. As mentioned above, the rotational speeds of the low-pressureshaft and of the high-pressure shaft (NL or NH, respectively) vary withthe differences in air bleed applied to either system. This variation isdependent of the type of control applied (speed of low-pressure shaft,NL or pressure ratio across the engine (thrust parameter, EPR)). Thus,according to the present invention, the regime of resonant vibrations isleft.

[0029] The difference in the pressure loss by tapping of bleed air whichis required can most simply be effected by individually setting thethrottle valves available within the system. In extreme cases, onesystem is closed off completely while the other is left open. To a minorextent, it is also possible to cool the bleed air within the fanair-operated heat exchanger to a different degree. Accordingly, thedifferent tapping of bleed air provides for a degree of freedom in termsof the optimization of the desired parameters (NL, NH, FN (net thrust)).

[0030] In the following, the changes proposed in the present inventionare explained in light of three, typical flight phases. The tables showextreme cases for bleed air distribution between the two engines,starting with a typical value for the tapping of bleed air. The columnheaded “normal” shows the values applicable to the tapping of equalquantities of bleed air from both engines. The extreme case—doublequantity of bleed air tapped from one engine, no bleed air tapped fromthe other engine—is shown in the columns headed “abnormal” and “none”

[0031] As a result of maximum air bleed, thrust will undergo variouschanges, these being due to the “EPR bleed air debits” provided in thecalculation (EPR=pressure ratio across the engine (thrust parameter)).This uneven thrust distribution creates a yaw moment which is eitherdesired or which must be corrected. In the first case, ΔNH obtained willbe larger. In the simplest case, the yaw moment can be avoided bydispensing with the EPR debits. This characteristic, i.e. constantthrust, was approximated in the examples by using a constant NL in thecalculation.

[0032] The results of the “normal” headed columns are the starting pointfor the calculations of EPR and NL controls. In the case of EPRcontrols, thrust, NL, NH etc. will change with air bleed. In the case ofNL controls, NL and consequently thrust, by approximation, will remainunchanged, while NH will change. TABLE 1 Take-off, EPR control Take-offEPR control max. Bleed air normal abnormal none delta Remarks LP bleedair 0.5 1.0 0.0 1.0 Typical value [lb/s] EPR [−] 1.4991 1.4861 1.51440.0283 Net Thrust FN 11616.6 11372.1 11902.6 530.5 Average thrust =11637 lbf, [lbf] i.e. 20.4 lbf higher sfc [lb/(lbf * s)] 0.4829 0.48580.4801 0.0057 Average sfc unchanged SOT [K] 1499.2 1495.6 1504.5 8.9 Inthe worst case, one engine is operated 5.3 K hotter than normal NL [rpm]6644.8 6599.0 6695.6 96.6 NH [rpm] 14894.5 14866.5 14929.3 62.8

[0033] TABLE 2 Take-off, NL control NL control max. Bleed air normalabnormal none delta Remarks LP Bleed air 0.5 1.0 0.0 1.0 Typical value[lb/s] EPR [−] 1.4991 1.4993 1.4987 0.0006 Net Thrust FN 11616.6 11618.011615.3 2.7 Average thrust = 11616.7 lbf, [lbf] i.e. unchanged sfc[lb/(lbf * s)] 0.4829 0.4857 0.4801 0.0056 Average sfc = 0.4829 => +0.0%SOT [K] 1499.2 1505.6 1492.8 12.8 In the worst case, one engine isoperated 6.4 K hotter than normal NL [rpm] 6644.8 6644.8 6644.8 0 Setconstant to obtain constant thrust! NH [rpm] 14894.5 14906.7 14882.324.4

[0034] If the engine is not “derated”, SOT will increase during take-offby 5.3 K (or 6.4 K). A maximum ΔNL of 96.6 rpm and a maximum ΔNH of 62.8rpm can be achieved. TABLE 3 Cruise, EPR control Cruise EPR control max.Bleed air normal abnormal none delta Remarks LP Bleed air 0.5 1.0 0.01.0 Typical value [lb/s] EPR [−] 1.6786 1.6552 1.6997 0.0445 Net ThrustFN 3682.9 3575.9 3780.6 204.7 Average thrust = 3678.3 lbf [lbf] sfc[lb/(lbf * s)] 0.6521 0.6581 0.6470 0.0111 Average sfc = 0.65255 =>+0.07% SOT [K] 1453.9 1451.0 1456.3 5.3 In the worst case, one engine isoperated 2.4 K hotter than normal NL [rpm] 6793.4 6700.0 6883.3 183.3 NH[rpm] 14235.7 14202.2 14265.0 62.8

[0035] TABLE 4 Cruise, NL control NL control max. Bleed air normalabnormal none delta Remarks LP Bleed air 0.5 1.0 0.0 1.0 Typical value[lb/s] EPR [−] 1.6786 1.6806 1.6759 0.0047 Net Thrust FN 3682.9 3683.23682.0 1.2 Average thrust = 3682.6 lbf [lbf] sfc [lb/(lbf * s)] 0.65210.6599 0.6442 0.0157 Average sfc = 0.65205 => +0.0% SOT [K] 1453.91465.2 1442.5 22.7 In the worst case, one engine is operated 11.3 Khotter than normal NL [rpm] 6793.4 6793.4 6793.4 0 Set constant toobtain constant thrust! NH [rpm] 14235.7 14253.9 14216.5 37.4

[0036] If the engine is not “derated”, the SOT of one engine duringcruise will increase by 2.4 K (or 11.3 K). A maximum ΔNL of 183.3 rpmand a maximum ΔNH of 62.8 rpm can be achieved. TABLE 5 Approach, EPRcontrol Approach EPR control max. Bleed air normal abnormal none deltaRemarks HP Bleed air 0.5 1.0 0.0 1.0 Typical value [lb/s] EPR [−] 1.01321.0128 1.0136 0.0008 Net Thrust FN 732.9 712.4 752.0 39.6 Average thrust= 732.2 lbf [lbf] Sfc [lb/(lbf * s)] 1.1785 1.2360 1.1275 0.1085 Averagesfc = 1.1818 => +0.27% SOT [K] 972.1 988.0 957.8 30.2 In the worst case,one engine is operated 15.9 K hotter than normal NL [rpm] 2898.8 2876.92918.4 41.5 NH [rpm] 11598.0 11598.0 11598.0 0 Since controlled to HI,NHRT26 = const

[0037] During approach, HI is automatically selected, which means thatcontrol is performed to NHRT26; consequently, the calculation here doesnot indicate a change in speed. Although HI is selected, control isfrequently assumed by another control law (e.g. min P30) and can,therefore, be overridden by another parameter just as well, i.e.selection of bleed air, cf. table 6. TABLE 6 Approach, NL control NLcontrol max. Bleed air normal abnormal none delta Remarks HP Bleed air0.5 1.0 0.0 1.0 Typical value [lb/s] EPR [−] 1.0132 1.0138 1.0133 0.0006Net Thrust FN 732.9 733.1 734.2 1.3 733.7 lbf [lbf] sfc [lb/(lbf * s)]1.1785 1.2138 1.1429 0.0709 Average sfc = 1.17835 => −0.013% SOT [K]972.1 989.7 956.0 33.7 In the worst case, one engine is operated 27.6 Khotter than normal. NL [rpm] 2898.8 2898.8 2898.8 0 Set constant toobtain constant thrust! NH [rpm] 11598.0 11634.3 11560.7 73.6 Onlypossible, if not controlled to HI.

[0038] The increase of sfc with EPR control (more precisely HI controlin this case) is quite irrelevant since this flight phase is relativelyshort. Also, the severe increase of SOT is not dramatic since it takesplace from a low starting basis. The small changes in thrust, whileprobably not being verifiable physically, are assumed to arise frominaccuracies in the calculation program (iterative process).

[0039] As becomes apparent from the above, the present inventionprovides for measures which enable the development of noise andvibrations to be positively influenced by changing the criticalexcitation frequencies directly at the source, i.e. the engine, and byshifting them towards an uncritical frequency.

[0040] Accordingly, the noise level in the entire area of the cabin willbe significantly reduced, in particular near the location of theengines. Furthermore, less attenuation material will be required, whichallows the mass of the aircraft to be reduced. The present invention canbe implemented by minor changes to the fuselage of the aircraft, thisresulting in a very low overall investment. Additionally, thepossibility to dispense with, or minimize, rudder trimming will resultin reduced fuel consumption and, accordingly, in a larger range.

[0041] Summarizing, then, the present invention relates to the exchange,the take-off or addition of media and/or power between the individualshafts of an engine, between individual engines and between the enginesand the aircraft. Thus, the present invention provides for additionaldegrees of freedom enabling engine parameters to be addressed in termsof a reduction or avoidance of negative resonances or beats.

[0042] The present invention relates to any number of engines on anaircraft and to any number of engine shafts. In accordance with thepresent invention, hydraulic power, electric power or air bleed can beinfluenced, for example.

[0043] List of Abbreviations

[0044] EPR Pressure ratio across the engine (thrust parameter)

[0045] FN Net thrust

[0046] ISA International standard atmosphere

[0047] NH High-pressure shaft speed

[0048] NHRT26 Aerodynamically corrected high-pressure shaft speed

[0049] NL Low-pressure shaft speed

[0050] sfc Specific fuel consumption

[0051] SOT Total entry temperature at the high-pressure turbine

[0052] HI High Idle

[0053] HP High Pressure

1-26. (Cancelled)
 27. A method for the control of at least one engine ofan aircraft having at least two engines, wherein an amount of at leastone of energy, fluid and other media that is at least one of supplied toand taken from the engine, is altered to alter thrust from that engineto alter trimming of a rudder of the aircraft.
 28. The method of claim27, wherein at least one of the energy, fluid and other media is takenfrom the engine.
 29. The method of claim 28, wherein the energy iselectric energy.
 30. The method of claim 28, wherein the fluid is bleedair.
 31. The method of claim 28, wherein the fluid is hydraulic fluid.32. The method of claim 27, wherein at least one of the energy, fluidand other media is supplied to the engine.
 33. The method of claim 32,wherein the fluid is bleed air.
 34. The method of claim 32, wherein thefluid is hydraulic fluid.
 35. The method of claim 32, wherein the energyis electric energy.
 36. The method of claim 35, wherein the engine is afully electric gas turbine.
 37. The method of claim 27, wherein at leastone of the energy, fluid and other media is taken from the engine and atleast one of the energy, fluid and other media is supplied to a secondengine.